It is known to provide a linear actuator such as a hydraulic cylinder between a vehicle body end of a lateral link or a trailing arm of a wheel suspension unit for supporting each rear wheel and the vehicle body, and to individually control the toe angles of the right and left wheels by extending or retracting the corresponding linear actuators (see Japanese patent publication No. 8-25482 and Japanese patent laid open publication No. 9-30438).
In such a system, a fail-safe mechanism is required so that the vehicle may be able to continue to travel without any inconvenience even in case of a failure of the system. For instance, the hydraulic actuator described in Japanese patent publication No. 8-25482 is provided with a piston rod that is resiliently urged by a spring member in the direction to retract the piston rod in such a manner that the toe angle is automatically restored to a zero angle when the hydraulic pressure is lost.
However, according to this prior art, when extending the piston rod under a normal condition, the hydraulic pressure is required to be high enough to oppose the spring force which is capable of maintaining the toe angle against the reaction from the road surface in case of a failure of the actuator so that the actuator and hydraulic pressure generator are required to be relatively large in size.
When the actuator is based on the use of a threaded rod, and the lead angle of the thread is selected in such a manner that the actuator remains immobile owing to the friction, because the linear displacement of the actuator is impossible without turning the threaded rod, there is some difficulty in providing a fail-safe mechanism.